Typical NMOS self-scan arrays use a multiplexing switch to serially address charge packets generated by their photodiodes and transfer these charge packets to the output node--a senseline. Upon the arrival of a charge packet, the senseline experiences a drop in voltage proportional to the size of the charge packet. The video signal consists of these voltage drops alternated with a recharge of the senseline to maintain a DC level. These NMOS self-scan imaging arrays are the low cost alternatives to more complex process oriented CCD imagers, but they have the following limitations:
1. Large minimum detectable light signal due to the large sense line capacitance.
2. Slow speed operation, typically 0.5 mHz data rate, due again to the large sense line capacitance.
3. Low dynamic range due to very large clock coupling to the output signal through the multiplexing switch.
4. Blooming due to light signal leakage through the multiplexing switch.
These problems are usually overcome by adding an array of dummy diodes so that noise can be differentially filtered out and by using sensitive off-chip amplifiers to read small charge packets. Although this improves the utility of self-scan arrays, these arrays have not found wide application because of the above-identified limitations.
According to the present invention, a sensor is described with a unique, indirect charge sensing scheme and a sense line capacitance neutralization circuitry. A new and unique sensing technique is disclosed which bypasses the limitations of conventional techniques by sensing current rather than charge. Each charge packet is paired with a current whose magnitude is controlled by the size of the charge packet. Although the currents are sequentially attached to the sense line via a multiplexing switch, the current passes over this node to be sent on a low capacitance node. As a result, the high capacity of the sense line is isolated from the signal detection and photogenerated charge packets are now able to be read. In addition, the currents controlled by the charge packets immediately sweep out any charge injected into the sense line by clock coupling through the multiplexing switch, thereby allowing a larger dynamic range.